1. Field of the Invention
The subject invention generally pertains to variable air volume air-handling units and more specifically to a mixed-air unit that includes a bypass damper.
2. Description of Related Art
An air handler is a piece of equipment for providing conditioned air to a comfort zone, such as a room or designated area within a building. In many cases, the conditioning of the air includes cooling, dehumidifying, and ventilating. For cooling and dehumidifying, an air handler typically includes a fan or blower that moves air across a cooling coil, which reduces the sensible and latent heat energy of the air. To balance the needs of both energy efficiency and ventilation, air supplied by the air handler is often a mixture of fresh outside air and a portion of return air from the comfort zone. The remaining portion of return air, not returned to the air handler, is typically exhausted outside.
For a complete air handling system, air handlers are typically associated with other components, such as ductwork, thermostats, dampers, air valves for regulating the airflow to the comfort zone, a chilled water supply for the cooling coil, a water valve for regulating the chilled water flow through the cooling coil, and a control unit. Air handler systems can usually be classified as either variable air volume (VAV) or constant volume systems.
Constant volume systems modulate the temperature of a comfort zone by providing the zone with a generally constant volume of airflow, while varying the temperature of the airflow to meet the zone""s sensible cooling demand. To do this, the flow of chilled water through the air handler""s cooling coil can be regulated by a valve in response to a thermostat associated with the zone. Although constant volume systems are quite effective at both cooling and dehumidify when operating at full load, high humidity problems can occur at certain part load conditions.
For instance, as the comfort zone""s demand for sensible cooling approaches a minimum, the water flow through the cooling coil is throttled back accordingly. This increases the temperature of the cooling coil, which in turn increases the temperature of the air being supplied to the comfort zone to meet the lower sensible cooling demand. However, the cooling coil operating at a higher temperature removes less moisture from the supply air. Thus, the humidity of the comfort zone increases.
VAV systems, on the other hand, supply air at a more constant temperature, and the flow rate or volume of supply air is modulated to meet the sensible cooling demand of one or more comfort zones. In cases where there are several zones, each zone may have its own thermostat controlling its own VAV valve. To meet each zone""s sensible cooling demand, the various VAV valves individually modulate the volume of supply air delivered to their respective zones. In addition, a control typically modulates the output of the VAV system""s main supply air blower according to the extent to which the various VAV valves are open. In other words, as the individual VAV valves throttle back the air supplied to their respective zones, the main supply air blower is controlled to deliver less air as well. Although such systems work fine over a broad range of conditions, in some cases, problems occur when the sensible cooling demand of one or more zones is satisfied or is at a minimum.
For example, if one zone requires cooling and a second zone of the same system does not, the VAV valve of the second zone will close to a minimum. However, to provide every room with at least a minimally acceptable amount of ventilation (often determined by industry standards or government codes), none of the VAV valves will close completely. Thus, cool air will continue to be supplied to that second zone, thereby possibly cooling the second zone beyond comfort.
One solution to such a problem is to locally reheat the air just before it enters the second zone. However, this not only wastes energy, but also often violates various government regulations.
An alternate solution employs a central control responsive to the position of the various VAV valves. The control resets the otherwise constant temperature of the supply air to a slightly higher temperature whenever one of the VAV valves is at its minimum open position. Raising the temperature of the supply air usually involves adjusting a water valve to reduce the amount of chilled water being conveyed through the coil. The temperature of the supply air can be incremented repeatedly until all of the VAV valves are open beyond their minimum position. Unfortunately, repeatedly incrementing the supply air temperature starts emulating a constant volume system where the supply air temperature is adjusted to meet the load. Thus, at very low sensible cooling loads, VAV systems can have some of the same humidity problems as constant volume systems.
Moreover, VAV systems responding to low sensible cooling loads with reduced supply airflow can increase humidity in another way. Government regulations require that buildings be supplied with at least a predetermined minimum amount of fresh outside air. When an air handler delivers a high volume of air to meet a high cooling load, perhaps only thirty percent of the air is outside air to satisfy the minimum fresh air requirement. The rest of the supply air originates from return air from the comfort zones. On the other hand, at minimal load conditions, the air handler delivers a much lower total volume of supply air, yet the amount of outside air remains the same. Thus, at minimal load conditions, the air handler delivers a much higher percentage of outside air. If the outside air is more humid than the building""s indoor air, which is often the case, then additional humidity is added to the indoor air during low sensible load conditions with traditional water valve control systems.
Consequently, a need exists for an air handler that can effectively and efficiently cool and ventilate a comfort zone at low load conditions, and do so with improved dehumidification.
To improve an air handler""s effectiveness at reducing humidity, it is an object of the invention to provide an air handler with a damper system that allows some return air to bypass the air handler""s cooling coil under certain operating conditions.
Another object of the invention is to provide an air handler that under moderate to high sensible load conditions provides supply air at a constant temperature and varying volume, and under low load sensible conditions provides supply air at a higher temperature and low humidity ratio by having some return air bypass the cooling coil. This also effectively provides some free reheat.
Yet another object is to provide an air handler with two return air mixing chambers: one upstream of the cooling coil and another downstream.
A further object of the invention is to provide an air handler with a damper system that not only allows some return air to bypass the cooling coil, but also ensures a predetermined minimum amount of fresh outside air, regardless of whether the air handler is operating under high or low load conditions.
A still further object of the invention is to improve an air handling system""s ability to dehumidify, wherein the system includes several VAV valves and a VAV blower.
These and other objects of the invention are provided by an air handler with improved dehumidification, especially at low load conditions. At low load conditions, the air handler with its cooling coil operating under moderate to high load conditions provides supply air at a constant temperature and varying volume. At low load conditions, the air handler provides supply air at a higher temperature with some return air bypassing the cooling coil.